The present invention relates to archery bows and, more particularly, to improvements which provide a faster and more accurate delivery of an arrow with a compound bow.
One existing bow design is the medieval long bow. In order for a long bow to be effective, it must be relatively long--about 6 feet. These bows can be readily manufactured from available material such as wood, but consequently are sensitive to humidity and temperature changes.
Another existing bow design which performs better in some respects than the long bow is the recurved bow. This type of bow has S-shaped or "recurved" limbs attached to either side of a rigid handle. When the limbs are made from appropriate laminate materials, a relatively short and still highly efficient bow can be made. However, the extent of recurvature is limited due to undesirable twisting of the limbs. Also, like the long bow, traditional recurved bows do not provide a way to hold an arrow in a drawn position without excessive fatigue of the user. U.S. Pat. No. 4,018,205 to Meyer provides illustrations and further detailed discussion about conventional long bows and recurved bows.
In response to the shortcomings of the simple long bow and recurved bow, the compound bow was developed. The compound bow offers several mechanical advantages over traditional straight and recurved bows. By and large, compound bows store more energy than non-compound bows. Also, a compound bow is generally more compact in terms of size for a given energy storage capacity.
Compound bows use a pulley system to provide a property called "let off." Let off results when the force required to hold the bowstring at full draw is substantially less than the force required to hold the bowstring in an intermediate position between the undrawn and fully drawn positions. Upon release of a bowstring which has been loaded with an arrow, the force propelling the arrow at a given position while in contact with the bowstring is proportional to the force required to hold the bowstring stationary in that position. Thus, in a compound bow, the arrow is subjected to a higher acceleration at an intermediate position during release than generally possible with a traditional bow of the same holding force at full draw. As a result, the archer is subjected to lower stress while aiming at full draw than for traditional bow designs.
Referring to FIG. 1A, a conventional compound bow 1 is illustrated. Generally, compound bow 1 comprises handle 2 connected to a pair of oppositely disposed bow limbs 30. A let off pulley system 10 including bowstring 20 is attached to each bow limb 30 and interposed therebetween. Typically, an arrow (not shown) is loaded along arrow path axis 8. Energy to propel a loaded arrow upon release is stored in each bow limb 30 by pulling bowstring 20 from the undrawn position shown in solid lines to the fully drawn position represented in phantom in FIG. 1A. The pair of bow limbs 30 act as springs which store energy when flexed by drawing bowstring 20.
Handle 2 is configured for gripping and includes arrow rest or ledge 3 upon which an arrow for shooting is placed. Handle 2 includes a pair of oppositely disposed limb seats 5 configured to receive mounting portion 32 of each bow limb 30. Each of the pair of screws 6 attaches a corresponding bow limb 30 to a corresponding limb seat 5 of handle 2.
Each of the pair of bow limbs 30 extends from handle 2 rearwardly towards bowstring 20. Each bow limb 30 has tip portion 34 opposing mounting portion 32. Each tip portion 34 is positioned outward from handle 2. Each bow limb 30 has inner edge 36 opposing outer edge 38 along its length. Also, each tip portion 34 corresponding to a bow limb 30 is connected to pulley system 10.
Pulley system 10 includes a pair of wheels 16 each correspondingly mounted to a bow limb 30 by one of a pair of pins 18. Also, pulley system 10 includes cables 12 and bowstring 20 attached between the pair of wheels 16. Cables 12 are also attached to each bow limb 30 by anchor 14. Each wheel 16 rotates or pivots about a rotational axis along corresponding pin 18. Wheel 16 includes cam sections which cooperate with cables 12 and bowstring 20 to provide let off when bowstring 20 is fully drawn. For more details concerning various let off pulley systems, see U.S. Pat. Nos. 4,739,744 and 4,515,142 to Nurney and 4,519,374 to Miller which are hereby incorporated by reference.
Referring to FIGS. 1B and 1C, bow limb 30 is depicted prior to assembly into bow 1. Notably, bow limb 30 is generally flat and straight prior to assembly. Mounting portion 32 defines aperture 32a adapted to receive a corresponding screw 6 therethrough. Bow limb 30 has flares or shoulders 33. Tip portion 34 defines slot 35 between arms 34a and 34b. Slot 35 is configured to receive one of the pair of wheels 16 for mounting therein. Arm 34a defines bore 35a, and aligns with bore 35b defined by arm 34b. Bores 35a, 35b are configured to receive pin 18 for pivotably mounting each wheel 16 to tip portion 34.
Referring specifically to the side view of FIG. 1C, it should be noted that bow limb 30 has thin portion 39 in between mounting portion 32 and tip portion 34. Typically, bow limb 30 is initially a rectilinear blank which is formed by removing material along edge 36. Notably, upper edge 38 remains generally straight even after thinning.
Referring back to FIG. 1A, it should be noted that when assembled into bow 1, bow limb 30 is restrained in a bent configuration between handle 2 and pulley system 10. Notably, thin portion 39 corresponds to the most severe degree of curvature in the bent bow limb 30 when assembled into bow 1. Each bow limb 30 bends even further in the fully drawn position.
One problem which remains with a conventional compound bow, such as bow 1, is that a considerable amount of energy stored in bow limb 30 is wasted by propelling the bow limb 30 forward when drawn bowstring 20 is released. Instead, it is desirable to use at least a portion of this wasted energy to propel an arrow. Force vectors F1 and F2 of FIG. 1D represent the force corresponding to each of the pair of bow limbs 30 at the point of release of a drawn bowstring 20. F1 and F2 are resolved into components along perpendicular coordinate axes x and y. Notably, the y axis generally corresponds to the bowstring 20 and the x axis generally corresponds to the arrow path axis 8 shown in FIG. 1A. Due to the general symmetry of bow 1 about axis 8, y components F1.sub.y and F2.sub.y are of approximately equal magnitude, but are oriented in opposite directions. As a result, the y components of F1 and F2 generally cancel each other. However, the x axis components F1.sub.x and F2.sub.x have generally the same direction; and so represent the force propelling bow limbs 30 forward when bowstring 20 is released with an arrow from the fully drawn position.
Furthermore, this forward motion of each bow limb 30 often causes handle 2 to jerk forward. Sometimes handle 2 even jumps from the archer's hold. These motions usually cause deviations in the flight path of an arrow. In fact, to improve accuracy, archers often minimize confinement of the handle 2 at the moment of release of an arrow through the use of a specially adapted wrist strap to loosely retain the bow.
Another type of conventional compound bow uses recurved limbs. FIGS. 2A and 2B illustrate a typical recurved bow limb 60 prior to assembly. Bow limb 60 has mounting portion 62 defining a mounting aperture 62a similar to aperture 32a of bow limb 30. Bow limb 60 has a tip portion 64 defining a slot 65 configured to receive a wheel. Slot 65 has arms 64a, 64b each of which define a bore 65a, 65b aligned with one another, respectively. Bore 65a, 64b are configured to receive a pin for pivotably mounting a wheel in slot 65. Bow limb 60 has flares or shoulders 63.
Also, bow limb 60 has recurved portion 70 with a point of inflection 72. Notably, recurved portion 70 has a reverse of curvature about inflection point 72. Bow limb 60 also has a thin portion 69 coinciding with recurved portion 70. Similar to bow limb 30 in FIG. 1A, a pair of bow limbs 60 are opposingly mounted to a handle with inner edge 66 closer to the bowstring than outer edge 68. A wheel is mounted with a rotational axis along bore 65a and 65b for each bow limb 60. Notably, the inflection point 72 lies along bow limb 60 between mounting portion 62 and bores 65a, 65b used to mount a wheel. One recurved compound bow design is shown in U.S. Pat. No. 4,712,533 to Cruise which is hereby incorporated by reference.
A compound bow with recurved bow limbs suffers from the same problems caused by forward motion of the bow limb upon arrow release as a compound bow with flat limbs. For both conventional limb types, once the bow limbs are attached to the handle, the corresponding tip portions generally align with an axis along the length of the handle prior to assembly with a pulley system. This generally straight configuration provides a practical limit on the degree of bow limb bending when assembled with a pulley system. This limitation permits substantial bow limb deflection in a direction parallel to the arrow path upon release of a fully drawn bow. Thus, a need remains to reduce the energy expended in propelling the bow limbs forward. Furthermore, at least some of this wasted energy should be redirected into the arrow to increase its speed.